This invention relates to the area of code division, multiple access (CDMA) wireless communication systems, and, more specifically, to decoding signals by a novel maximum likelihood sequence estimation decoder.
According to the IS 95 CDMA standard, a base station transmitter receives communication content and translates the content into symbols. The symbols are expanded by a spreading factor to a greater number of chips (the basic measurement of time in a CDMA system) and the chips are scrambled using an access code. The signal is then transmitted along with many other signals on the same frequency. The signal propagates through a dispersive medium and arrives at a receiver via multiple paths (also known as channels), each having a different propagation delay. As a result, each signal is received at the receiver overlapping with copies of itself, and each copy is delayed by one or more chips.
In most radio communications systems, such multi-path signals are out of phase with each other, which causes the signal to interfere with itself, resulting in a drop in received energy and concomitant loss of signal quality. In a CDMA system, in theory, these multi-path signals are added together to increase the signal strength, thus raising the quality of the received signal. To this end, several receivers are used simultaneously in a configuration known as a xe2x80x9crakexe2x80x9d receiver, where each receiver.is a xe2x80x9cfingerxe2x80x9d of the xe2x80x9crake.xe2x80x9d Each finger is timed to receive a different multi-path signal. The signal received at each finger of the rake receiver is delayed to align with the last arriving signal. All of the signals are then added together to increase signal strength. Thus, a rake receiver is designed to receive signals that have propagated through multi-path channels and rely on the so-called processing gain of the combined multi-path signals to suppress interference between the multiple paths.
While the signal strength may be improved by processing gain, symbols may still be incorrectly interpreted because the frequency of the signal may have changed due to multi-path and other well known effects (i.e., Doppler shift, etc.). In some systems, previously-decoded data symbols are used to establish what the channel error (also called xe2x80x9cfrequency shiftxe2x80x9d or xe2x80x9cfrequency errorxe2x80x9d) must have been immediately after decoding the last data symbol and immediately prior to decoding the next data symbol, thus forming a coherent reference. However, coherent references formed in this manner are inaccurate if a previously decoded symbol was in error, leading to error propagation. Therefore, IS 95 transmits a pilot code that is modulated only with known symbols to permit receivers to obtain a coherent reference that is independent of the unknown data symbols being decoded. The pilot code method is only suitable if the pilot code and all information bearing coded signals are transmitted from the same antenna.
Future CDMA systems may wish to exploit smart antenna beam forming by creating a directive beam specific to each signal. Then each signal must carry known symbols with which to establish a per-beam coherent reference. If the proportion of known symbols to unknown symbols is too small, then the coherent reference will be noisy; on the other hand, if the proportion of known to unknown symbols is too large, then the overhead creates inefficiency. This deficiency is resolved if an improved method of using unknown symbols is used to refine the coherent reference or channel estimates.
Prior art rake receivers for Code Division Multiple Access signals are described, for example, in U.S. Pat. No. 5,305,349 to Applicant entitled xe2x80x9cQuantized-Coherent Rake Receiverxe2x80x9d and in U.S. Pat. No. 5,572,552 to Dent and Bottomley, entitled xe2x80x9cMethod and System for Demodulation of Downlink CDMA Signals,xe2x80x9d both of which are incorporated herein by reference. Prior art rake receivers, while being designed to receive signals that have propagated through multipath channels, assume that the processing gain upon despreading the signal is sufficient to suppress interference between the multiple paths. Moreover, prior art coherent rake receivers assume a means to obtain a coherent reference that is independent of the unknown data symbols being decoded. For example, in the U.S. CDMA cellular system based upon the IS 95 standard, base stations transmit a special coded signal known as the pilot, specifically intended to provide a coherent reference for the mobile rake receivers. In U.S. Pat. No. 5,187,619 to Applicant entitled xe2x80x9cCDMA Subtractive Demodulationxe2x80x9d which is herein incorporated by reference, and in the already incorporated ""349 patent, previously decoded data symbols are used to establish what the received signal phase must have been immediately after decoding the last data symbol and immediately prior to decoding the next data symbol, thus forming a coherent reference. However, coherent references formed in the latter manner are inaccurate if a previously decoded symbol was in error, leading again to error propagation.
Other data symbol-assisted methods to provide coherent references exist in the prior art. In particular, for non-CDMA signals, Gudmundson describes, in U.S. Pat. No. 5,164,961, how to determine coherent references for all possible sequences of a limited number of successive symbols and how to use those references in deciding which sequences to retain as decoded sequences. In Gudmundson, later symbol decisions are not used to improve the measure of likelihood for the decoded symbol sequence. However, in U.S. Pat. Nos. 5,557,645 and 5,619,533 to Dent, entitled xe2x80x9cChannel Independent Equalizer Device,xe2x80x9d a method is disclosed whereby the likelihood measure for the decoded symbol sequence is periodically updated to be the value it would have been had the most recent symbol decisions been available from the beginning. Neither the Dent or Gudmundson patents, however, disclose how to apply those techniques to CDMA signals, which is an objective of the invention described below.
When a successive decoder such as described in the above ""961, ""645 and ""533 patents are decoding fading signals, there can be advantages in dynamically selecting between forward and reverse time order for decoding, as disclosed in U.S. Pat. No. 5,335,250 to Applicant, which is herein incorporated by reference.
Pilot symbol-assisted methods to provide coherent references exist in the prior art. In non-CDMA systems, coherent references are determined for all possible sequences of a limited number of successive symbols, which are then used in deciding which sequence to retain as the decoded sequence. In one such system, the likelihood measure for the decoded symbol sequence is periodically updated to the value it would have been had the most recent symbol decisions been available from the beginning.
This invention is directed toward overcoming one or more of the problems set forth above.
In accordance with one aspect of the invention, a method for decoding symbols in a received signal is disclosed for use in a receiver that receives signals carrying symbols over a plurality of channels, wherein the symbols are selected from a predefined set of symbols. The receiver has a memory storing a plurality of previously hypothesized sequences of symbols and a plurality of cumulative vectors, each describing one of the sequences of symbols. The method comprises the steps of generating a plurality of correlated vectors by correlating the received signal over a plurality of time offsets equal in number to the plurality of correlated vectors, the plurality of time offsets corresponding to selected ones of the plurality of channels, and generating a set of new symbol vectors by combining each of the plurality of correlated vectors with each one of the set of symbols. The method further includes hypothesizing a plurality of extended sequences of symbols by combining each of the plurality of new symbol vectors with each of the plurality of cumulative vectors and determining the most likely sequence of symbols by combining the previously hypothesized symbols and the one of the set of symbols corresponding to the combined cumulative vectors and new symbol vectors with the longest sum square length.
In accordance with another aspect of this invention, the step of generating a set of new symbol vectors comprises multiplying each of the plurality of correlated vectors with each one of the set of symbols. Further, the step of determining the most likely sequence of symbols comprises selecting the set of symbols by maximum likelihood sequence estimation. In accordance with another aspect of the invention, the method further includes the step of storing the selected sequence of symbols as one of the previously hypothesized sequence of symbols and storing the combined cumulative vector and new symbol vector with the longest sum square length as the cumulative vector describing the one of the previously hypothesized sequences of symbols.
According to another aspect of this invention, an improved receiver is disclosed for decoding spread-spectrum coded signals received through multiple propagation channels to obtain decoded information symbols. The receiver comprises a converter for filtering, amplifying, sampling and converting received signals to representative numerical samples, a correlator for correlating the numerical signal samples with a despreading code over each of the information symbol periods to obtain, for each symbol period, complex correlations for different time-alignments between the despreading code and the signal samples, wherein each time-alignment corresponds to a different one of the multiple propagation channels. The receiver further includes hypothesizing means for generating data symbol hypotheses, combining means for combining the symbol hypotheses with the complex correlations to obtain channel estimates, complex vector accumulation means for accumulating the channel estimates corresponding to the same one of the multiple propagation channels for successive symbol hypotheses to obtain cumulative complex vectors and a selector that selects sequences of symbol hypotheses for which the sum of the squares of the lengths of the cumulative complex vectors is greatest.
According to another aspect of this invention, the hypothesizing means is a Viterbi Maximum Likelihood Sequence Estimator, and the complex vector accumulation means includes means for de-weighting of older accumulated channel estimates. The de-weighting may include subtracting means for subtracting the contribution of an oldest channel estimate from the respective cumulative complex vector. Alternatively, exponential de-weighting may be used.
According to still yet another aspect of this invention, the frequency-error compensation means includes means for compensating for phase drift and frequency error, which may include means to rotate the phase angle of previous cumulative complex vectors before accumulating a new channel estimate.
In accordance with another aspect of this invention, the complex vector accumulation means includes compensation means to compensate for inter-symbol interference when the complex correlation for one symbol period is affected by the symbol value in at least one adjacent symbol period. Further, the complex vector accumulation means includes means to compensate for inter-ray interference in which the correlations for one path depend on the parameters of the other paths. The complex vector accumulation means may also include means to compensate the channel estimates before accumulation for the effect of other paths and adjacent symbols.
According to another aspect of this invention, the combining means may multiply the correlations by the complex conjugate of a symbol hypothesis. The spread-spectrum coded signals may comprise pilot symbols already known to the receiver interspersed with information symbols not already known to the receiver. The symbol hypotheses may be a single hypothesis for each of said known pilot symbols combined with multiple hypotheses for each of said unknown information symbols. Further, the number of combined symbol sequence hypotheses retained by the receiver falls to one upon receiving a predetermined number of pilot symbols.
In accordance with another aspect of this invention, an improved receiver is disclosed for decoding spread-spectrum coded signals received through multiple propagation paths to obtain decoded information symbols. The receiver includes a converter for filtering, amplifying, sampling and converting received signals to representative numerical samples and a correlator for correlating the numerical signal samples with a despreading code over each of the information symbol periods to obtain, for each symbol period, complex correlations for different time-alignments between the despreading code and the signal samples. Each time-alignment corresponds to a different one of the multiple propagation paths. The receiver further includes hypothesizing means for hypothesizing data symbol sequences, channel estimation means for forming channel estimates for each of the multiple propagation paths as a function of the complex correlations and the hypothesized symbol sequences and decision means to select the most likely of the hypothesized sequences using the channel estimates, thereby decoding the information symbols.
In accordance with another aspect of this invention, the hypothesizing means is a Maximum Likelihood Sequence Estimator. Further, the channel estimation means may multiply hypothesized symbols with corresponding complex correlations and form a sum of the results, wherein one of the multiplicands is complex-conjugated and the sum may be a weighted sum. The weighting may assign a lower weight to older symbols and a higher weight to more recent symbols and may include frequency and/or phase error compensation.
According to another aspect of this invention, an improved receiver is disclosed for decoding error-correction encoded, interleaved and spread-spectrum modulated signals received through multiple propagation paths to recover information symbols. The receiver includes converter means for filtering, amplifying, sampling and converting received signals to representative numerical samples and correlation means for correlating the numerical signal samples with a despreading code over each of the spread-spectrum modulation symbol intervals to obtain, for each modulation interval, a corresponding set of complex correlations for different time-alignments between the despreading code and the signal samples. Each time-alignment corresponds to a different one of the multiple propagation paths. The receiver further includes information symbol hypothesizing means for hypothesizing sequences of information symbols, error-correction coding means for coding the hypothesized symbol sequences to produce coded modulation symbol sequence hypotheses, de-interleaving means for selecting sets of the correlations corresponding to successive ones of the hypothesized modulation symbols, likelihood estimating means for combining the selected sets of correlations in order to produce a likelihood indication for each of the hypothesized information symbol sequences and selection means for selecting an information symbol sequence having the greatest likelihood indication.
According to another aspect of this invention, the information symbol hypothesizing means may be a Maximum Likelihood Sequence Estimator.
According to yet another aspect of the invention, the likelihood estimating means forms a cumulative estimate of the complex signal amplitude for each of the multiple propagation paths, wherein the cumulative estimates are formed separately for regions of the received signal samples that are widely separated in time.
In accordance with another aspect of this invention, a method is disclosed for decoding spread-spectrum modulated symbols received by multiple propagation paths, including known symbols interspersed with unknown data symbols. The method includes the steps of pilot-filtering, amplifying, sampling and converting received signals to representative numerical samples, correlating the numerical signal samples with a despreading code over each of the spread-spectrum modulation symbol intervals to obtain, for each modulation interval, a corresponding set of complex correlations for different time-alignments between the despreading code and the signal samples. Each time-alignment corresponds to a different one of the multiple propagation paths. The method further includes the steps of initializing a Maximum Likelihood Sequence Estimator to a single starting state including a likelihood indication using sets of correlations corresponding to known pilot symbols interspersed among unknown data symbols, successively hypothesizing unknown data symbols within the Maximum Likelihood Sequence Estimator and combining each new data symbol hypothesis with a corresponding set of correlations and the previous state or states to obtain an expanded number of states. Each of the expanded number of states corresponds to hypothesized sequences extended by one more unknown data symbols and each has an associated likelihood indication. The method includes the steps of selecting sequences having the highest likelihood indications in order to keep the number of states at or below a maximum allowed number of states, combining sets of correlations corresponding to pilot symbols interspersed after the unknown data symbols with the previous state or states in order to obtain a contracted number of states corresponding to a reduced number of hypothesized symbol sequences and selecting from the reduced number of symbol sequences the hypothesized symbol sequence having the indication of highest likelihood, thereby decoding the unknown symbols.
In accordance with another aspect of the method, pilot symbols may be denoted as being interspersed before or after unknown data symbols using a reversed time convention and decoding of some symbols may use only the reversed time convention and decoding of other symbols uses the normal time convention.